This is a competitive renewal of a proposal that was designed to test the guiding hypothesis that the stress-related neurohormone, corticotropin-releasing hormone (CRH), serves as a neurotransmitter to activate the major brain noradrenergic nucleus, locus coeruleus (LC) during stress. Our previous studies provided anatomical and physiological evidence supporting this hypothesis. Additionally, these studies suggested certain circuits by which endogenous CRH activates the LC, and certain physiological consequences of this activation. The following AIMS extend these previous studies: Aim 1 will characterize the anatomical substrates underlying CRH regulation of the LC at an ultrastructural level using electron microscopy and dual labeling immunohistochemistry. It will be determined whether CRH terminals directly contact LC cells and/or LC dendrites that extend into the pericoerulear region, or terminals of other afferents to the LC. Sources of CRH inputs to the LC and pericoerulear region will be confirmed using electron microscopy and double labeling of terminals for CRH and anterograde tract tracers. This approach will also be used to identify other LC afferents that CRH may interact with. Aim 2 will identify specific CRH systems that regulate LC activity during different types of stress. The impact of activating identified CRH afferents to the LC or pericoeulear region on LC discharge will be characterized, and the role of neurotransmitter CRH in these effects will be determined. By combining retrograde tract tracing from the LC with immunohistochemistry for CRH and immediate early gene proteins, CRH afferents to the LC that are activated by specific stressors will be identified. Aim 3 will test the hypothesis that a consequence of LC activation by endogenous CRH during stress is suppression of cell-mediated immunity. The effects of CRH microinfusion into the LC on spleen T-lymphocyte mitogenic activity will be quantified and the role of stress-elicited CRH release in the LC in suppression of cell-mediated immunity will be determined. Taken together, these Aims examine the CRH-LC connection in stress at both an input (ultrastructural and systems level) and an output (functional) level. CRH-brain noradrenergic interactions are thought to be involved in adaptive and pathological consequences of stress. As stress is thought to be a major contributing factor to a number of psychiatric disorders, these studies will facilitate our understanding of the etiology of stress-related psychiatric disorders, their treatment and prevention.